<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    .completed label{
      text-decoration:line-through;
    }
  </style>
</head>
<body>
  <ul>
    <li>
      <label>吃饭</label>
    </li>
    <li class="completed">
      <label>睡觉</label>
    </li>
    <li>
      <label>打豆豆</label>
    </li>
  </ul>
</body>
</html>